In the prior art of the invention, a variety of large aperture zoom lens of four or five lens groups have been proposed, and especially, some of them serve as a standard zoom lens suitable to a single-lens reflex camera. Above all, many types of the large aperture zoom lens with a wide angle of 70 degrees or higher are a negative lead zoom lens where a group of lens of negative refractive power leads ahead of the remaining lens groups. A typical one of them is a zoom lens having four groups of lenses of negative, positive, negative and positive refractive powers in the order on the closer-to-the-object first basis, and such zoom lens is large in aperture diameter and still attains the improved performance (for example, Japanese Patent Laid-open Nos. 7-43613 and 2000-221399).
Another type of the large aperture zoom lens proposed in the art is that which has four groups of zoom lenses having positive, negative, positive and positive refractive powers in the order on the closer-to-the-object first basis (e.g., see particularly Japanese Patent Laid-open No. 10-197794). The zoom lens is available from the assignee of this application in the model number 176A (having features of 28 to 105 in focal length and 2.8 in F number).
For recent years, it has been a trend to develop a large aperture zoom lens of a wider angle of field while attaining the more and more improved performance. Some of this type of the large aperture zoom lens takes an inner focusing system where only the second group of lenses are moved for the focusing. This system is advantageous in that, since there is no need of moving more massive and heavier group of lenses like the first lens group, a motor drives the lenses with the reduced burden during the automatic focusing to enable a quick adjustment for the focusing and that an effective diameter of the front lens or the first group of lenses can be retained relatively small, and this effectively leads to the downsizing.
Compared with a general zoom lens in the state of the art, however, the large aperture zoom lens is still bulky and heavy. In addition, as a result of the development of the compact and lightweight single-lens reflex camera body, the prior art large aperture zoom lens is not well-balanced with the camera body and its portability is unsatisfactory. The large aperture zoom lens must receive more luminous fluxes to enhance the brightness in contrast with the standard zoom lens of a comparatively large F number, and this essentially brings about a larger lens diameter. At the same time, the required aberration compensation becomes complicated to necessitate the larger number of lenses, and this increases the total length of the lenses, which adversely leads to increased dimensions of the lens as a whole.
The model Number 176A related to Patent Document 3 has fifteen lenses and four aspherical surfaces. It is 112.0 mm in total length and φ 82 mm in filter diameter when it shrinks in shortest formation. Thus, compared with the standard zoom lens of a relatively high F number (i.e., that belongs to a class that contains lenses 28 to 105 in focal length and 3.5 to 4.5 in F number), the model number 176A is 37 mm longer in total length and 20 mm greater in filter diameter.
The large aperture zoom lenses related to Patent Documents 1 and 2, compared with the model number 176A, have the total length as much as or greater than the total length of the latter in its shortest formation, and this is still bulky. The model number 176A is yet still excessive in dimensions in use for the standard zoom lens as well.
The present invention is made allowing for the aforementioned disadvantages of the prior art large aperture zoom lens, and accordingly, it is an object of the present invention to provide a downsized and lightweight, and highly portable large aperture zoom lens.
It is another object of the present invention to provide a large aperture zoom lens of the improved optical performance, serving as a standard zoom lens especially suitable to a single-lens reflex camera, which employs a zooming system with four groups of lenses having positive, negative, positive and positive refractive powers, respectively, and which implements on the order 75 degrees of a photographing angle of field at a wide-angle end of the range, approximately 2.8 of F number throughout the zooming range, and about 2.8 of a variable power of magnification while still dimensioned as close as the standard zoom lens for the single-lens reflex camera (i.e., about 95 mm in total lens length and φ67 mm in filter diameter on the side closer to an objective lens).